The present invention is directed to a method for manufacturing a composite material racket frame. More specifically, the present invention is directed to the fabrication of a racket frame having an outer shell of resin impregnated fiber sheet and an inner core of foamed material.
There are two typical methods for fabricating a racket frame having an outer shell of resin impregnated fiber sheet and an inner core of foamed material One of these two methods is known as the self-bonding single-step molding process. In the single step method, a core element including foamable material is first inserted into an outer shell fabricated from a multi-ply sheet of resin impregnated fiber. The resulting structure is placed in a racket frame mold. The mold is then heated to foam the foamable material, and to mold the resin impregnated fiber sheet. As a result of pressure generated from the expansion of the core, the uncured outer shell is forced to contact with the inner surface of the mold thereby taking the shape of a racket frame. As it is difficult to control the uniformity of the generated pressure and the core and shell cannot simultaneously receive the heat which is transferred from the heated mold, the foaming action of the foamable material can not match the gel time of the curing thermosetting resin. If the foaming action is too late, the shell will set before it takes the shape of the mold. If the foaming action takes place too early, it causes an extreme pressure to build up and the core composiition will penetrate into the shell, thereby damaging the fiber texture of the resin impregnated fiber sheet, and weakening the endurance of the racket to the stroke.
The second of these two typical methods is known as the two-step molding method. In the two-step method, an outer tubular shell of thermosetting material surrounding a one end closed cellophane tube is placed into a mold of a racket frame. Compressed air is blown into the cellophane tube through its open end to force the outer tubular shell into contact with the inner surface of the mold so as to cause the outer tubular shell to take the shape of the racket frame. After the mold is cooled and the cured outer shell is taken out of the mold, the closed end of cellophane is cut open. After cutting open the cellophane, a foamable solution is introduced into the cured outer shell while heating it so as to foam the foamable material to form a shock absorbing core. This two-step method has an operational disadvantage in that an extra step of cutting the closed end of cellophane has to be carried out before introducing the foamable solution. In addition a higher production cost cannot be avoided because removal of cured racket frame from the mold can only be done after complete cooling of the mold. Moreover if, for blowing the foaming material the racket should be heated to a temperature higher than the heat distortion temperature of the outer shell, without the support of the mold the outer shell will be deformed due to the pressure generated from the foaming action.